Process of manufacturing photoelectric cells



Oct. 7, 1952 J. M. J. DUBAR ETAL 2,613,301

PROCESS OF MANUFACTURING PHOTOELECTRIC CELLS Filed Jan. 6, 1950Selena/12.9, f A? 4 /ola 017 Pla'nzmz.

/ezppl'ef' Jaya? 'aamlmz, op Zinc, op HIM/22mm, 0P412122012y,

IN VEN TORS e012 Jules MgJ 5650/1: @ahw /Ue M5566.

and Hagel Piz Patented Oct. 7, 19h52 PROCESS F MANUFACTURINGPHOTOELECTRIC CELLS Lon Jules Marie Joseph Dubar, Paris, and RogerPhilippe Millet, "Pavillons-'sous-Bois, France, assignors to Compagniedes Freins et Signaux Westinghouse, Paris, France Application January 6,195.0,fSeriai No. 137,301'` In FrancehJanuai-y 17, 1949 A barrier layerphoto-electric cell consists, like a dry rectifier, of a thin layer of asemi-conductor. interposed between two metallic electrodes. In therectifiers one of the electrodes, the base plate, serves as a mechanicalsupport Afor the semi-conductor, with which as perfect as possible anelectrical connection is made. The phenomena of asymmetric conductivityare localized at the contact of the semi-conductor and the secondelectrode, called the counterelectrode. It ,i is agreed that thisasymmetry is due to the existence, at the interface between the metal ofthe counterelectrode and the semi-conductor, of` a very thin layer ofhigh resistivity, called the bar-v rierlayer, which isfformed in thecourse of the manufacturing treatment.

trons in the direction from the metal to the semi-conductor, than -inthe reverse direction. In consequence, the direction of the lgreaterconductivitycorrespon'ds to theflow of current `from the base plate tothe counterelectrode` through the semi-conductor. A

The photo-electric cell differs from the rectifier in that thecounterelectrode, consisting, for

example, of a thin layer y,of gold or platinum, allows light to passwhile'lrhaining conductive. If the surface-of the semi-conductor isilluminated through the counterelectrode, the appearance of anelectrical difference of potential may be observed between it andthebaseplate; the positive pole being the base plate.` Thisdif-I ference ofpotential on open 'circuit is not'proportional to the illumination butincreases lessrapidly and appearsto tend toward a saturation'v `issensibly proportional to the illumination. :Since the cell constitutes arectifier, .it will be -seen that the photo-electric current which flows:from the negative pole to `the positivej pole `through the cell, thatis to say, fromthe counter- Yelectrode to the base plate, ilows in thehigh resistance direction through the rectifier.

These phenomena may beexplained, as for the rectifier, by the existenceof a barrier layer between the semi-conductor and the counterelectrode.The light, traversing the latter, liberates some electrons in the'semi-conductor. Those which have suiicient energy traverse the barrierOn This layer istraversed much more easily by the conducting 'elec- 2equilibrium is established between the electrons passing from thesemi-conductor to the counterelectrodel and those which recombine withtheir ions in taking the reverse path, and a sort of saturationintervenes. On short circuit, on the contrary, this manifestationpractically does not appear, the current forcing the electrons to moveall together and they recombine with their ions after having entered theexterior path including the load circuitl and the base'plate. Underthese conditions, the current delivered is proportional to the number ofelectrons liberated per second inthe semi-conductor by the primaryphotoelectric e'ect. Y 'Y It will be seen'that on the one hand theuseful photo-electric current traverses the barrier` layer inthedirection corresponding to the blocking resistance of the contactbetween the semi-conductor and the counterelectrode; on the other handthe currents of direct recombination of electrons with l their positiveions take the reverse path, which is that of the low resistance of thecontact.` However, in addition to this recombination, which hasv itslocation vin a space very close to the barrier layer, there exists someleakage'currents which are not negligible which flow directly from thecounterelectrode to the base plate perhaps clue to defects in thesurface, or through the non-illuminatedportions of the cell (collectorring, for example). Finally, certain 'photo-electrons, which, moreover,are more numerous when the illumination is very intense, iiow directlyfrom the semi-conductor to the base plate. In effect, the semi-conductoris not rigorously opaque and the region of emission of photoelectronsapproaches closer to the base plate 4 when the illumination is veryintense.

These different defects contribute each in part to the diminution of thepotential difference on open circuit with strong illumination. Now it isevident that the power capable of being furnished by the. cell to a loaddevice will moreover be greater, for the same short circuit current, ifthe difference of potential of the cell on open circuit,

whichL may be likened to an apparent electrometive force,` is increased,under the condition that theapparent resistance of the load apparatus isadapted to the apparent internal resistance of thecell.

The present invention, due to the Work of Messrs. `Roger Millet and `LonDubar, obtains cells.- of delinitely increased electromotive force byreduction of the leakage currents of which the origin will be explainedthus: This result is obtained according to the invention by creating abarrier layer between the semi-conductor and the base plate. Thus asecond rectfying contact is obtained, in opposition to the rectifyingcontact of the semi-conductor and counterelectrode. This back contactallows however the useful photo-electric current to pass easily andnotably reduces the leakage currents in the reverse direction. Moreover,it eiciently blocks the photoelectrons, strongly retarded by theirpassage through the semi-conductor. which are emitted directly towardthe-'base plate.

By way of example, and not as a limitation, as applied to the seleniumcell, the'use-may be indicated of a base plate of a metal capable `oiforming with the selenium a non-conducting compound. This metal can be,for example, cadmium, zinc, aluminum, antimony, etc. use'das a solidplate or an electrolytic deposit. The 'barrier layer is formed byassociation of the selenium with the metal after the heat treatmentwhich transforms the selenium to a conducting variety. The duration andthe temperature. of this heat treatment can be regulated to obtain theoptimum properties at the same time for the selenium and the barrierlayer, the other treatments remaining unchanged.

Experience has shown that besides a notable increase in the differenceof .potential on open circuit, which can reach up to 0.5 volt at 1.200lux, there is obtained with certainy metals such as cadmium a remarkableadherence of the layer of selenium to the base plate sufficient toenable optionally the. omission ofthe usual roughening of the base plateby a sand blast. This property should likewise be considered as one ofth results accomplishedb'y the invention.

It is obvious that theA example of realization given is not limitedbythe invention but may be applied to any barrier-layer cell whether itis of selenium or not and, nally, that the secondbarrier layer. betweenthe semi-conductor and the base plate or back layer can be realized inany known manner. Thisconsi'sts of a deposit on the base plate, previoustothe addition thereto of the semi-conductor, of an appropriateinsulating film, organic (such as varnish) or mineral (such as quartz..fluorite, cryolite, the deposit being made by evaporation in vacuum).

Moreover, the invention by no meansv depends upon the theoreticalconsiderations disclosed above. servation of the increase in thedifference 4of potential on open vcircuit by means of strongillumination following the creation of a rectifying contact between thebase plate and the semiconductor and not to the explanations which maybe given for this result.

By reason of the present invention, having regard to the direction ofrecticati'on, the' useful photo-electric current is not sensiblyaffected while the undesired leakage currents are notably attenuated.The result is an increase in the internal resistance and in theelectromotive force of the cell permitting the latter to deliver alarger power to a receiver. 'Y

In the accompanying drawing, Fig.V 1l is la plan view, and Fig. 2 is across-section View of a photoelectric cell constructed according 'to ourinvention; and Fig. 3 is a view showing an enlarged portion of thecross-section view shown in Fig. 2.

Referring to the drawing, a base plate 2 is provided, of a metal whichcan be, for example, cadmium, zinc, aluminum, antimony, etc., used Itrests uniquely upon the actual obeither as a solid plate, or anelectrolytic deposit on a supporting base plate l, A layer 3 of asemi-conductor selenium material is then applied to the exposed or upperside of base plate 2, as shown in the drawing. By means of a heattreatment, in an oven at a temperature above centigrade but notexceeding the melting point of selenium, the layer 3 of semi-conductorselenium material becomes transformed into a conducting Variety.

A counterelectrode consisting, for example, of a thin layer 4 of gold orplatinum, is applied to the exposed side of selenium layer 3, and asuitable collector electrode, such for example as a ring shapedelectrode 5 may be applied to counterelectrode 4.

In Fig. 3, the usual barrier layer 6 is shown between selenium layer 3and counterelectrode 4. Asecond barrier layer 1 is also formed betweenthe base plate 2 and layer 3 of selenium. Barrier layer 1 has higherresistance to the flow of current. inI the direction from base plate 2to selenium layer 3 than in the opposite direction from selenium layer 3to base plate 2, whereas barrier layer 6, as is Well known, has higherresistance to the ilow of current in the direction from counterelectrodeylv to selenium layer 3, than from layer 3 tofcounterelectrode 4.

Having thus describedour invention, what we claim is:

A process of manufacturing a photo-electric cell which comprisesdepositing an organic film suchas varnish, or aiilm of a mineralselected.

from the'gro-up consisting. of quartz, uorite and v cryolite 'ony afbaseplate of a metal capable of forming a barrier layer witheterni-conductor material such asl selenium, then depositing a layer ofsaid semi-conductor material on said lm, providing av thin layer ofmetal such as gold or` platinuml in contact with the opposite side/of.said-.layer of ysemi-conductor material, Eimplying heat to transformsaid semi-conductor material into a conducting state, in which saidbarrier layer is formed between said base plate and said layer ofsemi-conductor material, said barrier layer having high resistance toelectric current of agiven `polarity and having low resistance tolelectric current of the opposite polarity, and in which La secondbarrier layer is formed betweenr said thin layer of metal and saidopposite side of said'A layer of semi-conductor material, said` secondbarrier layer having low resist-ance to'` electric current of said givenpolarity, andhaving high resistance to electric current of. saidopposite polarity.

LEONJULES MARIE JOSEPH DUBAR. ROGER. PHILIPPE MILLET.

REFERENCESCITED AThe following references are of record in the l'e' vofthis 'patenti t UNITED STATES PATENTS

